Both hypertrophy and atrophy of the heart activate the fetal gene program, while cardiac mass changes in opposite directions. This dichotomy leaves a critical gap to identify those pathways that convey directionality, because regression of left ventricular hypertrophy decreases the risk for cardiovascular morbidity and mortality. We have shown that mechanical unloading of the rat heart activates the ubiquitin proteasome pathway and decreases cardiac mass. In isolated myocytes we found that activation of the forkhead transcription factor FOXOSa increases the expression of two ubiquitin ligases (Mafbx/Atrogin-1 and MuRF-1) and decreases cardiomyocyte size. The overall hypothesis is that the activation of pathways of protein degradation in the unloaded heart induces reverse remodeling in the presence of pro-hypertrophic signals, both in animal models and in the failing human heart. The first specific aim will define the signaling pathways determining a decrease in cardiomyocyte size in vivo. The second specific aim will determine the reversibility of cardiomyocyte hypertrophy in vitro and in vivo by activating signaling pathways of protein degradation. In gain-of-function and in loss-of-function strategies we will define the role of the FOXO3a/ Mafbx/Atrogin-1/ MuRF-1 pathway in the regulation of gene expression, atrophy, and function in the hypertrophied rodent heart. The third specific aim will define signaling pathways regulating cardiomyocyte atrophy in the human heart. Here, we will examine signaling pathways regulating atrophy in the failing human heart before and after mechanical unloading with a left ventricular assist device. Because insulin is a potent negative regulator of myocyte atrophy (e.g. insulin decreases Mafbx/Atrogin-1 expression), we will examine mediators of atrophy in serial samples of left ventricular myocardium incidentally obtained during coronary artery bypass surgery in the presence and absence of glucose-insulin-potassium. Our long-term objective is to define transcriptional and post-transcriptional mechanisms that regulate regression of hypertrophy in the presence of pro-hypertrophic signaling and to translate findings derived from animals to workable clinical strategies.